P indeed used to be The Residence in the past (when it was a separate class of service, not just a seat option). Later, after The Residence as a class of service was discontinued, P was reassigned to discounted business but Flying Blue didn't update their earning chart.
I would expect FB indeed to post 400% miles since their system will see P class and thus should award miles for P class. But be prepared for 0 as a possibility, especially if retrocredit is needed (which often is the case with EY).
I have an EY P flight coming in late November, will report afterwards.
(Regarding the debate above, I'm not sure how Lufthansa's P class is relevant, since EY is neither *A, nor LHG, and in fact most LHG airlines do credit miles for P
)